INTRODUCTION

ABSTRACT

There is a complex network of ~800 miles (1280 km) of above ground pipelines in one of the largest oil fields on the North Slope of Alaska. This infrastructure has been in operation for nearly three decades, transporting crude oil, water and gas. The vast majority of the piping network is insulated with polyurethane foam over bare steel, and is therefore susceptible to corrosion under insulation (CUI). The sheer magnitude of the number of locations susceptible to CUI poses integrity management challenges that call for a better understanding of damage onset and progression rates as well as underlying causes. Radiographic testing, visual inspection, and refurbishment data were analyzed for ~73 miles (117 km) of pipelines where complete external corrosion inspection histories were available. The analysis showed that the accumulated damage distribution over time could be fit to Weibull curves, which appear to correlate with the total number of CUI repairs and refurbishments that need to be done each year. The Weibull curves generated were employed to predict the number of CUI susceptible locations that would need repair or refurbishment each year for all 800 miles of pipelines in the field. The model can be further used as input to risk assessments to support maintenance strategies, and as background rationale for budgeting and manpower optimization for inspection, repair and refurbishment activities.

Corrosion under insulation (CUI) is a major facet of modern asset integrity management in the oil and gas industry, which is facing challenges in extending the use of aging infrastructures beyond their originally envisioned service lives. Industry aims for zero hydrocarbon leaks and spills given our concern for safety and health, environmental impacts, corporate core values, increased regulatory oversight, and potential damage to industry reputation caused by heightened public awareness of corrosion-related incidents1-4. This aim is pursued in a context of limited financial resources and limited qualified personnel, driving a search for continual improvement in strategies that translate to workable tactics and optimal resource prioritization.

The oil and gas industry has recognized the significance of CUI as well as approaches to detect and mitigate the degradation with added emphasis since the 1980s5. Risk-based strategies founded on API 580 etc. are largely qualitative or intuitive, leading to broad categorization of assets to rank susceptibility to CUI founded on service conditions6-8. Inspection techniques using radiography, long-range ultrasonics, and thermography have been used in detecting wet insulation and underlying corrosion therein6, 7, 9. A major emphasis in mitigation of CUI is via coatings10, 11, which have found their way to regulatory stipulations4.

Some limitations of the current approaches taken by the industry are, a) risk-based inspection (RBI) applied without consideration of site specific inspection and maintenance histories severely limits the effectiveness of risk management, b) corrosion detection often occurs late in the life cycle of CUI in which wet insulation is the precursor or a leading indicator and c) many lines lack an external coating which could provide a reasonable and effective barrier to CUI.

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